The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Single wall corrugated high density polyethylene (HDPE) pipe was first developed and has been in existence since the 1970's. Such single wall plastic pipe was first formed in small diameters (e.g., 3″ diameter) as a cost effective and productivity enhancing replacement for clay tile used in agricultural applications. Single wall corrugated HDPE pipe allows for significant installation efficiencies due to its flexible nature and the fact that it is available in long and coiled lengths. The longitudinal flexibility and coilable nature of the pipe permits substantially automated plowing of long lengths of the pipe into the ground, as opposed to installing individual sections of pipe through open trench excavation.
However, single wall corrugated pipe does have its drawbacks. For instance, single wall corrugated HDPE is not hydraulically efficient relative to other non-coilable pipe. For gravity flow applications, the interior roughness of pipe is measured by the Mannings coefficient. Single wall corrugated HDPE has a relatively rough interior surface, with a minimum Manning's coefficient falling in the range of 0.021-0.030. Moreover, as technology for installing single wall corrugated HDPE has progressed, so have the number of field failures for corrugated HDPE. The newer installation technology for single wall agriculture applications involves plowing the pipe in the ground. However, with the newer technology, contractors have the tendency to axially stretch the pipe, which results in a reduction in structural properties. The axial stretching of the pipe has led to numerous field failures.
As the corrugated HDPE industry progressed, the diameter ranges of corrugated single wall pipe increased from 3″ up to 24″, allowing for similar efficient installation practices for larger diameter pipe. Additionally, with further advancements in manufacturing technology, dual wall corrugated HDPE pipe was soon developed. Dual wall pipe has the same corrugated exterior but a smooth wall interior, resulting in dramatic improvements in fluid flow capacity and performance. Dual wall corrugated HDPE has a typical Manning's coefficient falling in the range of 0.010-0.015, which makes it more hydraulically efficient than single wall corrugated HDPE. Additionally, dual wall corrugated HDPE's smooth interior is less likely to get clogged with silts and sands that are in the water being transported in the pipe. Dual wall corrugated HDPE pipe was first introduced into the market on or about in the mid 1980's.
Corrugated dual wall HDPE pipe exhibits many of the same characteristics as single wall HDPE pipe, such as strength and lightweight construction, but also offers significantly increased flow capacity due to the smooth inner wall. Moreover, when tested for axial pipe stiffness in accordance with ASTM Standards (i.e., ASTM F405), the contents of which are incorporated herein by reference, corrugated dual wall pipe has superior strength. Corrugated dual wall HDPE has sufficient axial strength to resist stretching the pipe in an axial direction, thereby preserving its strength when mis-installed by contractors. By contrast, conventional current-day single wall corrugated HDPE pipe does not have sufficient axial strength to keep from being stretched during the installation process.
In addition to the above, with higher axial stiffness, corrugated dual wall HDPE pipe has allowed for the development of a bell and spigot type coupling system. Increased axial stiffness associated with the inner liner has enabled the assembly of a bell and spigot pipe configuration utilizing a compression fit. The compression fit caused by an O-ring style gasket is useful in keeping silts out of the pipe. Such a compression fit typically requires about two lbs. per inch of nominal diameter (2.0 lb/in dia.) axial compression force to engage a bell and spigot coupling system. Corrugated single wall HDPE pipe, on the other hand, utilizes a split coupler that wraps around the outside of the pipe, or an internal snap coupler. This is necessary because the axial stiffness of corrugated single wall pipe is insufficient to permit the compression fit associated with the bell and spigot type coupling system. Both corrugated single wall HDPE pipe joints are considered inferior to the bell and spigot coupling system used with dual wall corrugated HDPE pipe.
The drawback with HDPE dual wall pipe, however, which persists to this day, is the longitudinal stiffness of such pipe caused by the presence of the smooth inner liner wall and general inelasticity of the material; its inability to flex longitudinally prevents many of the installation efficiencies provided by the single wall pipe design. Conventional current-day corrugated HDPE dual wall pipe cannot be coiled or flexed longitudinally without breakage. Consequently, this has required a significant change in installation practice from plowing pipe into the ground to open trench excavations, a practice which is far less efficient and significantly more costly.
Open trench excavation may be appropriate and accepted for certain civil construction applications due to the high expectations for installed performance and the relatively small amount of pipe required to be installed on any single project. For agricultural applications, however, along with flow capacity, installation efficiency is the primary concern. Usually, there are only narrow windows of time between spring thaw and spring planting, and between harvest and ground freezing, during which installation is reasonably practicable, and most projects require installation of thousands and tens of thousands of feet of pipe. Consequently, for agricultural projects, improving installation time and lowering installation cost has a significant impact on the overall project cost. The ability to install pipe via the use of automated plowing equipment is paramount, and the use of pipe having increased axial strength to prevent field failures would also help significantly to increase installation production rates. Because of this installation cost component, there has been a long-felt, strong and unsatisfied need for innovation in the area of high flow capacity corrugated HDPE pipe (i.e., corrugated dual wall pipe) which is flexible enough to be plowed into the ground, has sufficient axial strength to be plowed at higher rates, and is also flexible enough to be coiled.
Finding a solution to the foregoing problems, however, is further complicated by the fact that numerous other variables, such as installation temperatures, processing conditions, pipe diameter, pipe profile geometry, etc., have an effect on and may determine the needs of a particular application. The specific composition of material utilized in the construction of coilable corrugated dual wall pipe for one set of circumstances or application may vary dramatically from that of another. Therefore, the appropriate solution requires suitable versatility to accommodate variations for differing application requirements.